The present invention relates to anechoic chambers of the type that are used to measure radiation that is emitted by electrical equipment and the like. The Federal Communications Commission (FCC) imposes limits on the amount of radiation that can be emitted by certain classes of such equipment sold in the United States. As a result, equipment must be tested to insure compliance with these FCC requirements.
Testing of such equipment must be performed over an exemplary frequency range of from a low frequency of 30 megahertz (MHz) to a high frequency of 1000 MHz in an open field environment, i.e. in an environment that is essentially free of radiation reflection and of ambient electromagnetic fields. In most cases, an open field environment is not available. As a result, tests are usually conducted in a closed anechoic chamber that is constructed and arranged to provide a quasi-open-field environment that is essentially free of radiation reflection and of ambient electromagnetic fields.
Anechoic chamber walls are conventionally lined with a material that absorbs electromagnetic wave energy, much as black paint operates to absorb light. U.S. Pat. No. 4,477,505 provides an example. The wavelength of light is quite small as compared to the wavelength of radiation in the 30 to 1000 MHz range. In general, anechoic chamber absorber material must be at least 1/4 wavelength thick for appreciable wave absorption to take place. Since wavelength is inversely proportional to frequency, the effective thickness of an absorber must increase as the frequency in question decreases.
Ordinarily, in an anechoic chamber a single backing layer of homogeneous material underlies the array of pyramid cone absorbers. This backing material is typically identical to the pyramid material. This backing layer is mounted onto the chamber's metal wall, which wall operates to shield the chamber from external radiation.
Known anechoic chambers using pyramidal absorbers provide excellent broad band suppression of reflected waves in the high frequency region. Very low reflection occurs from the pyramidal absorbers at high frequencies since incident wave energy is reflected several times from the flat surfaces of the cones before finally being reflected back into free space. A fraction of the high frequency incident wave is absorbed at each one of these reflections or bounces. Thus, high frequency waves are very much diminished in strength or magnitude by the time they reflect from the array of pyramidal absorbers into free space.
This physical picture of absorption at high frequencies in valid because the wavelengths involved are relatively small in comparison to the physical size of the pyramidal absorbing structures. As a result, quasi optical reflections occur.
At low frequencies, however, the wavelength becomes much longer than the spacing between adjacent pyramidal absorbers. The depth of penetration of the electromagnetic wave ("skin depth") in the absorbing material likewise becomes long compared to the size of the pyramids. For such a pyramid structure, reflection at low frequencies cannot be modeled in terms of successive reflections from individual surfaces of the cones. Instead, it is more natural to consider an array of pyramids as an equivalent layer of material with a flat surface located in the X-Y plane.
Thus, at low frequencies, pyramidal absorbers can be simulated to some extent by layering a number of flat sheets of absorbing material, each layer having a slightly different permittivity and conductivity factors. Such an absorber is also called a gradient absorber. The greater the number of sheet layers, the more broad band will be the absorbing layer. Since cost must be considered, the number of flat sheets is generally limited to 5 or 6 sheets at the most, and most layered resonant material absorbers are made up of 3 layers. (MICROWAVES, December 1969, an article entitled "Microwave Absorbing Materials and Anechoic Chambers-- Part 1", beginning at page 38). On the other hand, such flat layered absorbers are far inferior to actual pyramid absorbers at high frequencies.
Low frequency electromagnetic waves propagate into and are absorbed by the array of pyramids just as they would be by any other material, with the exception that some of the incident radiation is scattered at angles other than that which is prescribed by Snell's law. Dominating such reflection is a pair of waves, i.e. the scattered and reflected waves, that propagate at the Snell reflection angles. The strengths or magnitudes of the scattered and reflected waves are determined by certain effective material characteristics of the above mentioned equivalent layer. For typical pyramid absorbers, the reflection coefficients for an array of such absorbers mounted on a metallic backing wall are quite large, tending toward unity at the low frequency limit. This makes pyramid absorbers of limited usefulness in anechoic chambers to be used at these lower frequencies.
The need for a low frequency chamber, however, is growing. Modern computing devices may emit in this spectrum, and compliance with FCC regulations requires that products be tested to prevent interference with communications. For example, in order to certify an anechoic chamber for such testing, the FCC requires that the fields within the chamber be within 4 dB of open field test conditions from 30 MHz to 300 MHz (the VHF band). While this requirement may not seem overly strict, it is somewhat difficult to satisfy, especially if the same anechoic chamber is also to be used at higher frequencies.
At lower frequencies, it is possible to achieve low reflection coefficients using a single layer dispersive absorber. Such absorbers can work well in a narrow range of frequencies, but because the dispersion of the absorbing material is not completely controllable, the bandwidth of these absorbers is narrow.
As mentioned above, one prior art approach to designing anechoic chambers has been the use of multilayer absorbers in the absence of pyramidal absorbers. These multilayer absorbers are built to minimize reflection in a specified range of frequencies. Prior to the present invention the design of multilayer absorbers required the use of interactive cut-and-try methods, Smith-chart methods, and numerical optimization techniques. In these designs, the layers are usually composed of homogeneous materials. This limitation narrows the achievable bandwidth for absorbers of a given size. A top layer continuously matched to the external medium has been shown to produce the best high frequency performance.
While anechoic chambers, and the methods by which such chambers have been designed, have been generally satisfactory in the art, the need remains to provide a means whereby the low frequency response of such chambers can be still further improved.